1. Field of the Invention
This invention relates to barrier coatings and the method for making the same. In particular, low stress, thick thermal barrier coatings are disclosed along with the method for making the coatings such that the thermal stresses in the coated substrate are relieved by providing a compliant ceramic layer between the substrate and a hard erosion resistant top coat layer.
2. Description of the Related Art
The drive for improved gas turbine engine performance is pushing hot section operating temperatures higher, typically, as high as 1600.degree. F. These temperatures have reached the point where metal alloys break down and means for protecting the metal are needed. In one commonly used method, the injection of cooling air from the compressor is injected into the turbine. However, the cooling air has a very negative effect on performance and efficiency. Therefore, alternative, ceramic thermal barrier coatings (TBC's) have evolved. Ceramics are chemically inert and remain strong at high temperatures. They also have the advantage of low thermal conductivity and, therefore, very effectively shield the metallic substrate layer.
In a conventional TBC's, the ceramic layer is bonded to a metal substrate by uniform application of a conventional plasma spray process. The difficulty with the uniform bonded ceramic layer, however, is that its thermal expansion coefficient is smaller than the substrate's. Differential strain across the layers during cool down following spraying or during nonequilibrium thermal excursions puts the ceramic into residual compression. This compressive stress produces edge shear which is greatest near the ceramic/substrate interface. If sufficient, the shear can cause in-plane spalling just above the metal/bondcoat layer corners of the sprayed parts where it is concentrated.
The interfacial shear stress increases with the elastic modulus of the ceramic layer as does the ceramic's resistance to erosion by particles passing through the engine. Thus, setting the spray process for a more erosion resistant ceramic layer may increase the stresses which cause spallation.
It is apparent from the above that there exists a need in the art for a TBC which relieves thermal stresses by providing a compliant ceramic layer between the substrate and a hard erosion resistant top coat layer. In this manner, the intermediate ceramic layer will be able to provide good thermal resistance while having a low effective modulus to longitudinal strains so as to reduce thermal stress in the turbine. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.